1. Field of the Invention
The present invention relates to a nonreciprocal circuit element such as an isolator or a circulator used for an antenna duplexer or the like.
2. Description of the Related Art
A conventional nonreciprocal circuit element will now be described with reference to FIGS. 13 to 15, in which FIG. 13 is an exploded perspective view of a conventional nonreciprocal circuit element, FIG. 14 is a plan view illustrating essential parts of the conventional nonreciprocal circuit element, and FIG. 15 is a developed view of a central conductor of the conventional nonreciprocal circuit element.
Referring to FIGS. 13 through 15, the conventional nonreciprocal circuit element includes a first box-shaped yoke 51, a disk-shaped magnet 52 disposed in the first yoke 51, and a flat ferrite member 53 disposed under the magnet 52. The conventional nonreciprocal circuit element further includes three central conductors 54, 55 and 56 formed of metal plates partly intersecting one another and mounted on the ferrite member 53 by being spaced apart from one another by an angle of 120 degrees, a box-shaped resin case 57 accommodating the ferrite member 53, and a second U-shaped yoke 58 disposed under the resin case 57.
The first box-shaped yoke 51 has a rectangular upper plate 51a, and four side plates 51b bent downward from four sides of the upper plate 51a. A magnet 52 is arranged within the first yoke 51, and its top surface is adhered to the inner surface of the upper plate 51a using an adhesive or the like.
The ferrite member 53 has a rectangular configuration in which two long sides 53a face each other and two short sides 53b face each other. The top surface of the ferrite member 53 is disposed to face the magnet 52 within the effective range of the bottom surface of the magnet 52.
As shown in FIG. 15, first, second and third central conductors 54, 55 and 56 are formed by notching metal plates and extend outwardly from a square ground portion 70 which is disposed in the center thereof.
Each of the first, second and third central conductors 54, 55 and 56 has two-divided conductors 54a, 55a and 56a each having a slit formed in a vertical direction, and first, second and third port portions 54b, 55b and 56b provided at ends of the conductors 54a, 55a and 56a, respectively.
The first, second and third central conductors 54, 55 and 56 are configured such that the ground portion 70 is disposed under the ferrite member 53. In such a state, the conductors 54a, 55a and 56a are bent along the side and top surfaces of the ferrite member 53.
The first, second and third central conductors 54, 55 and 56 are disposed on the top surface of the ferrite member 53 in a vertical direction in a state in which they are individually insulated from one another by insulators, although not shown.
When the first, second and third central conductors 54, 55 and 56 are mounted on the ferrite member 53, the first and second central conductors 54 and 55 are placed at the short sides 53b so as to traverse the longer surface of the ferrite member 53 while the third central conductor 56 is placed at the long side 53a so as to traverse the shorter surface of the ferrite member 53 in a state in which it is parallel to the short sides 53b. 
The resin case 57 has a bottom wall 57b having a rectangular hole 57a at its central portion. The bottom wall 57b has concave portions 57c formed along three sides of the hole 57a. The ferrite member 53 having the first, second and third central conductors 54, 55 and 56 mounted thereon is disposed within the hole 57a, so that the ground portion 70 corresponding to one end of each of the first, second and the third central conductors 54, 55 and 56 is connected to the second yoke 58.
Chip-type capacitors 62, 63 and 64 and a chip-type resistor 65 are arranged within the concave portions 57 around the hole 57a, and lower electrodes of the capacitors 62, 63 and 64 and an electrode 65a at one end of the resistor 65 are connected to the second yoke 58, respectively.
The respective port portions 54c and 55c of the first and second central conductors 54 and 55 are connected to the upper electrodes of the capacitors 62 and 63 by soldering, while the port portion 56c of the third central conductor 56 is connected to the upper electrode of the capacitor 64 and to the top surface of the electrode 65b at the other end of the resistor 65 by soldering.
In other words, the capacitor 62 is disposed in one of long sides 53a of the ferrite member 53, and the capacitor 63 is disposed at the short side 53b perpendicular to the long side 53a. Also, the capacitor 64 and the resistor 65 are formed at the other long side 53a so as to surround the hole 57a. 
In a state in which the magnet 52, the ferrite member 53 and the resin case 57 are interposed between the first and second yokes 51 and 58, the first and second yokes 51 and 58 are connected with each other to form a closed magnetic circuit composed of the first and second yokes 51 and 58, thereby completing the nonreciprocal circuit element, as disclosed in U.S. Pat. No. 6,597,257.
However, since the magnet 52 of the conventional nonreciprocal circuit element is accommodated in the box-shaped first yoke 51, it is necessary to reduce the size of the magnet 52 and the ferrite member 53 provided corresponding thereto.
As the nonreciprocal circuit element becomes smaller sized, the magnet 53 and the ferrite member 53 are further miniaturized accordingly. Consequently, the lengths of the two-divided conductors 54a, 55a and 56a of the first, second and third central conductors 54, 55 and 56 mounted on the ferrite member 53 are decreased, resulting in deteriorating the performance of the nonreciprocal circuit element.
Also, since the capacitors 62, 63 and 64 and the resistor 65 are arranged in the vicinity of the hole 57a including the ferrite member 53, the ferrite member 53 reduces in size. Further, since the lengths of the conductors 54a, 55a and 56a of the first, second and third central conductors 54, 55 and 56 reduce, the performance of the nonreciprocal circuit element may deteriorate.